1. Field of the Invention
The present invention relates to a load drive apparatus having a pulse drive circuit which repeatedly applies a pulse voltage to a resistive load, and to a sensor control apparatus including the load drive apparatus.
2. Description of the Related Art
Conventionally, a load drive apparatus which drives a resistive load such as a heater for heating a detection element of an oxygen sensor mainly composed of, for example, solid electrolyte such as zirconia includes a pulse drive circuit. One end of the resistive load is grounded, and the output terminal of the pulse drive circuit is connected to the other end of the resistive load. The pulse drive circuit repeatedly applies a pulse voltage to the resistive load. In such a load drive apparatus, the connection path between the pulse drive circuit and the resistive load may suffer a connection anomaly such as formation of a short circuit to the power supply potential or the ground potential or a break in the connection path.
Therefore, preferably, the load drive apparatus is able to properly detect the occurrence of such a connection anomaly.
For example, Patent Document 1 discloses a method and an apparatus for monitoring the operability of a heater of an oxygen measurement sensor. The apparatus and method are configured to measure the power supply voltage of the heater when the heater is operated and when the heater is stopped, and to output a corresponding fault signal when the voltage difference becomes less than a predetermined minimum value or when the voltage difference becomes greater than a predetermined maximum value.
The above-mentioned method and apparatus are based on the principle that the power supply voltage of the heater drops when the heater is operated (energized). Thus, the power supply voltage of the heater is measured when the heater is operated (energized) and when the heater is stopped (de-energized), and the state of a failure (wire breakage or short circuit) is diagnosed based on the measured voltage. Specifically, when a predetermined diagnosis condition is satisfied after waiting, the power supply voltage Uon of the heater is measured when the heater is operated (energized). After that, when the heater is stopped (de-energized), the power supply voltage Uoff of the heater is measured. In the case where the difference between Uoff and Uon is not greater than a minimum value ΔU1, a wire breakage is determined to have occurred in the heater. In the case where the difference between Uoff and Uon exceeds a maximum value ΔU2, a determination is made that the heater has been short-circuited. The diagnosis is also repeated when other diagnostic conditions are satisfied.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. H5-195843